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Original question:

ISO121 Single power supply

ISO121: ISO121 Single power supply

Clifton Chey
Prodigy 20 points
Part Number: ISO121
Other Parts Discussed in Thread: AMC1300, AMC1301

Hi,

My applicaiton is to measure a current-source (200nA to 10uA) of 50ohm, and the Isolation is to be 3KV and above. I have selected ISO121 as an Isolation Amplifier for the rated 5KVdc barrier.

My Questions:

1. ISO121 requires +-15V, and as the barrier is above 3KV, I have to use a DC/DC converter, which is very noisy. Can ISO121 supports single Power-Supply of +15V, and I can do away with the +-15V? I am currently using a DC/DC converter and the noise is about 40mV. As my input signal is small, I need the noise to be lower for good SN ratio.

2. The recommended Gain Adjust in Data-sheet, is an input of 1Kohm(R1), can this resistance be changed to 100kohm? Should the sense resistance be changed to 200kohm for a gain of 5?

Thanks

 

  • 0
    TI__Guru**** 172866 points
    Hi Clifton,

    Welcome to our e2e community! I'm sorry to tell you that the ISO121 will not operate properly with a single supply, you would need at least +/-4.5V on both +/-VS1 and +/-VS2 as noted on pages 1 and 2 of the datasheet. The gain of the ISO121 is also fixed at 1V/V (see GAIN on page 2) and the gain trim of figure 4b is only meant for small scale adjustments. A precision gain block ahead of the ISO121 would be your best option. You can also look at the AMC1300 or AMC1301 - these are newer isolation amplifiers and provide higher isolation ratings. They also operate from single supplies which might make your design a little easier.
  • 0 in reply to Tom Hendrick
    Prodigy 20 points
    Hi Tom,

    Thank you for your suggestions. I will implement a low-noise Opamp ahead of ISO121. AMC1300 or AM1301 has a much lower Isolation Voltage of <1500Vdc, which is not suitable to my requirements.

    Regards
    Clifton